The office has, for many years, been a stressful environment due, in part, to the large number of objectionable noise generators, such as typewriters, high speed impact printers, paper shredders, and other office machinery. Where several such devices are placed together in a single room, the cumulative noise pollution may even be hazardous to the health and well being of its occupants. The situation is well recognized and has been addressed by governmental bodies who have set standards for maximum acceptable noise levels in office environments. Attempts have been made by the technical community to reduce the noise pollution. Some of these methods include enclosing impact printers in sound attenuating covers, designing impact printers in which the impact noise is reduced, and designing quieter printers based on non-impact technologies such as ink jet and thermal transfer.
Noise measurements are often referenced as dBA values. The "A" scale, by which the sound values have been identified, represents humanly perceived levels of loudness as opposed to absolute values of sound intensity. When considering sound energy represented in dB (or dBA) units, it should be borne in mind that the scale is logarithmic and that a 10 dB difference means a factor of 10, a 20 dB difference means a factor of 100, 30 dB a factor of 1000, and so on.
Typically, impact printers generate impact noise in the range of 65 to just over 80 dBA, which is deemed to be intrusive. When reduced to the high 50s dBA, the noise is construed to be objectionable or annoying. It would be highly desirable to reduce the impact noise to a dBA value in the vicinity of 50 dBA. For example, the IBM Selectric ball unit typewriters generate about 78 dBA, while the Xerox Memorywriter typewriters generate about 68 dBA. The typewriter of the present invention has been typically measured at slightly less than 52 dBA. This represents a dramatic improvement on the order of about 100 times less noisy than present day offices, a notable achievement toward a less stressful office environment.
Although the printing impact, produced as the hammer impacts and drives the type character pad against the ribbon, the print sheet and the platen with sufficient force to release the ink from the ribbon, is the major source of noise in the typewriter, other noise sources are present. In the presently available typewriters, the impact noise overshadows the other noises. But, once the impact noise has been substantially reduced, the other noises will no longer be extraneous. Thus, the design of a truly quiet printer requires the designer to address reducing all other noise sources, such as those arising from platen vibration, carriage motion, character selection, ribbon lift and advance, as well as from miscellaneous clutches, solenoids, motors and switches.
In conventional ballistic hammer impact printers a hammer mass of about 2.5 grams is driven ballistically by a solenoid-actuated clapper toward the ribbon/paper/platen combination. When the hammer hits the rear surface of the character pad it drives it against the ribbon/paper/platen combination and deforms the platen surface which, when it has absorbed the hammer impact energy, seeks to return to its normal shape by driving the hammer back to its home position where it must be stopped, usually by another impact. This series of impacts is the main source of the objectionable impact noise.
Looking solely at the platen deformation impact portion of the hammer movement, the total dwell time is typically in the vicinity of 100 microseconds. At a printing speed of 30 characters per second, the mean time available between character impacts is about 30 milliseconds. The impact noise reduction achieved by the printing mechanism of the present typewriter is made possible by significantly stretching the impact dwell time to a substantially larger fraction of the printing cycle than is typical in conventional printers. For instance, if the dwell time were stretched from 100 microseconds to 6 to 10 milliseconds, this would represent a sixty- to one hundred-fold increase, or stretch, in pulse width relative to the conventional. By extending the deforming of the platen over a longer period of time, the resonant frequency is proportionally decreased and an attendant reduction in noise output can be achieved. In a resonant system, since the mass is inversely proportional to the inverse of the frequency shift, a massive increase in hammer mass is required.
The general concept implemented in the present typewriter, i.e. reduction of impulse noise achieved by stretching the deformation pulse, has been recognized for many decades. As long ago as 1918, in U.S. Pat. No. 1,261,751 (Anderson) it was recognized that quieter operation of the printing function in a typewriter may be achieved by increasing the "time actually used in making the impression". A type bar typewriter operating upon the principles described in this patent was commercially available at that time.